Candida albicans is ubiquitously present in the microflora of the body and its success as both a commensal and a pathogen relies on its ability to adapt to changes in host physiology. However, despite its prevalence as an opportunistic pathogen, the mechanisms contributing to phenotypic plasticity and pathogenesis are poorly defined. This proposal will establish the role of sexual reproduction for increasing phenotypic diversity and promoting infection of multiple sites in the mammalian host. In addition, it will explore if components of the sexual signaling pathway represent novel targets for antifungal therapies. C. albicans was originally thought to be obligately asexual but an efficient and elaborate mating program has now been established that can generate recombinant forms of the organism. Mating in C. albicans is unique in that it is regulated by phenotypic switching; only opaque forms of the organism can undergo sexual reproduction. In addition, meiosis has not been observed and instead cells undergo a program of concerted chromosome loss to complete a parasexual mating cycle. Progeny from the parasexual cycle are recombinant strains that exhibit divergent phenotypes with the potential for increased virulence in the host. This possibility will be addressed by analysis of a set of progeny strains using in vitro and in vivo assays. Many of the products of the parasexual cycle are aneuploid strains carrying extra copies of chromosomes. As drug-resistant isolates of C. albicans often exhibit changes in chromosome copy number our studies will also determine if parasexual progeny include those with increased drug resistance. Sexual reproduction in C. albicans and related yeast is thought to occur exclusively between a and a mating partners. However, preliminary experiments show a novel mechanism exists for same-sex mating within unisexual populations of the organism. Autocrine pheromone signaling is responsible for promoting self-mating and the mechanism of autocrine signaling will be established by genetic approaches. These studies also have important implications for mechanisms of sexual reproduction in related Candida species that propagate exclusively as unisexual populations. Finally, our studies will investigate if components of the sexual machinery can be targeted to promote killing of fungal cells. Preliminary experiments suggest that sexual pheromones act to reduce the cellular integrity of responding cells. The mechanism of signaling will be defined and the possibility tested that cell killing by common antifungal drugs can be potentiated by targeting of pheromone-signaling pathways.